Loadbearing Architectural Precast Concrete Wall Panels

Sidney Freedman Director Archi tectura l Precast Concrete Serv ices Precast/Prestressed Concrete Institute Chicago, illi nois

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Architectural precast concrete wall panels that act as loadbearing e le m e nts in a building are both a stru c turall y effic ie nt and economical means o f transferring floor and roof loads through the structure and into the foundation. In many cases, this integration can also simplify construction and reduce costs. This article presents the m an y be ne fit s that ca n b e deri ved from using lo adb ea rin g architectural precast concrete wa lls in buildings. Discussed herein are the va rio us shapes and sizes of wa ll pane ls, major des ign considerations, and when loadbearing or shear wall units should be the first design choice. The role o f connections, shear walls, and the use of precas t concre te as fo rms fo r cas t-in-place concre te is explained. In general, the design methods and techniques presented in this article apply to buildings in both seismic and non-seismic areas. The latter part of this article shows how these design principles can be applied in practice in a variety o f buildings. These examples illustrate the use of w indow wall panels, spandre ls, and solid or sandwich wall panels as the loadbearing wall members. When all the advantages o f using architectural precast concrete as loadbearing wa lls are added up, it makes good sense to use this structural form in building applications.

Loadbearing facades have both an aesthetic and structural function. In buildin g prac ti ce, the most

economical application of architec­tural precast concrete is as loadbearing structural elements. Loadbearing units become an integral part of the struc­ture, taking the vertical and horizontal floo r and roof loads, and/or transfer­ring horizontal loads into shear walls or service cores. Such an arrangement

can be economical, not only from a structural design standpoint, but also fro m the view point of overall co n­struction. In some cases, the loadbear­ing elements also can contribute to the horizontal stability of the building.

Arc hitec tura l precas t co nc rete cladding is noted for its diversity of express ion, as we ll as its desirable therma l, acousti c and fire -resistant properti es . Commonl y overlooked is

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(a) Flat, hollow-core, or insulated panel.

(b) Vertical window or mullion panel.

(c) Horizontal window or mullion panel.

--

(d) Ribbed panel. (e) Double-tee panel. (f) Spandrel (same as "a").

the fact that concrete elements nor­mally used for cladding applications, such as solid wall panels, window wall or spandrel panels, have considerable inherent structural capability.

In the case of low- or mid-rise struc­tures, the amount of reinforcement re­quired to handle and erect a precast component is often more than neces­sary for carrying imposed loads. Thus, with re lat ively few modifica tion s, many cladding panels can function as loadbearing members. For taller build­ings, additional reinforcement may be necessary for the lower level panels.

The slight increases in loadbearing wall panel cost (due to reinforcement

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and connection require ments) can usually be more than offset by the elimination of a separate perimeter structural frame. Depending upon the application, the loadbearing panels also may reduce or eliminate a struc­tural core or interior shear walls, par­ticularily in buildings with a large ratio of wall-to-floor area. The in­crease in interior floor space gained by eliminating columns can be sub­stantial and, depending on the floor plan, partition layout flexibility can be enhanced.

To take maximum advantage of load­bearing units, decisions as to their func­tions should be made before structural

Fig. 1. Various types of architectural loadbearing wall panels.

design has progressed to a stage where revisions become costly. Cost savings tend to be greatest in low- to mid-rise structures of three to ten stories. As with all precast concrete applications, further economies can be realized if the panels are repetitive. Besides minimiz­ing the number of casting forms neces­sary , repetitive panel designs enable repetitive connections.

Architecturalloadbearing panels can be used effectively to renovate and re­habilitate old deteriorated structures. These panels can be used not only in all-precast structures but also in struc­tural steel framed structures and cast­in-place concrete structures.

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Guidance for using loadbearing ar­chitectural precast concrete wall panels can be found in Refs . 1 and 2. Other pertinent information on this subject can be found in the list of references.

SHAPES AND SIZES Architectural load carrying compo­

nents can be provided in a variety of custom designed or standard section shapes. A wall system can be com­prised of flat or curved panels (solid, hollow-core, or insulated) (see Figs. 1 a and f), window or mullion panels (see Figs. 1 b and c), ribbed panels (Fig. ld) , or a double-tee (see Fig. le) . Each type of panel will readily accom­modate openings for doors and win­dows. Fig. I b, c, d and e illustrate var­ious types of ribbed panels. The panel shown in Fig.lc is a horizontal Yierendeel truss window mullion panel, while the other panels are verti­cal window mullion panels. Fig. If shows an exterior horizontal spandrel as part of a column-wall system.*

In the interest of both economy and function, precast panels should be as large as practical, while considering production efficiency and transporta­tion and erection limitations. By mak­ing panels as large as possible, numer­ous economies are realized - the number of panels needed is reduced, fewer joints (waterproofing require­ments), lower erection cost, and fewer connections are required. Panels may be designed for use in either vertical or horizontal positions.

For low-rise buildings, by spanning loadbearing panels vertically through several stories, complex connection details can be minimized, and conse­quently the economic advantages of loadbearing wall panels are increased.

For high-rise buildings, it is normally more practical to work with single­story horizontal panels connected at each floor level. The elements can be more slender, simplifying the erection.

Ref. 3 discusses the use of horizontal panel s on the 16-story United Bank Tower in Colorado Springs, Colorado (see Fig. 2). The single-story panels are typically 14ft 6 in. x 16ft x 8 in. (4.42 x 4.88 m x 203 mm) thick with 13 x 30

* In most cases, when the term "panel" is used in the

text, it refers to all panels shown in Fig. I.

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in. (330 x 762 mm) monolithically cast pilasters at the ends (see Fig. 3).

Multistory panels usually do not ex­ceed 45 ft (13.7 m) in height- the maximum transportable length in many states (see Fig. 4) . Panels should be designed in specific widths to suit the building 's modular planning. When such a building is designed properly, the economic advantages of loadbearing wall panels are signifi­cantly increased.

Fig. 2. United Bank Tower, Colorado Springs, Colorado.

Panel dimensions generally are gov­erned by architectural requirements. Most shapes, textures, and surface fin­ishes commonly associated with cladding are possible, provided struc­tural integrity and other technical re­quirements can be satisfied at the same time.

Load uniformity is one of the im­portant advantages for high-rise, load­bearing panel structures where the bearing walls also serve as shear

Fig. 3. Single-story wall panels.

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walls. It produces even loads on the perimeter foundations and reduces the tendency for differential settlement. The jointed nature of the facade also makes it more tolerant of any differen­tial settlement.

Curves are easily handled by precast concrete. On curved panels, a continu­ous supporting ledge cast on the inside face is preferred to provide bearing for floor/roof members and to stiffen the panels to minimize warpage. The Po­lice Administration Building in Philadelphia, Pennsylvania, made his­tory as one of the first major buildings to utilize the inherent structural char­acteristics of architectural precast con­crete (see Fig. 5). The building is un­usual in its plan configuration, consisting of two round structures connected by a curving central sec­tion , demonstrating the versatility of precast concrete for unusual plan forms. The 5 ft (1.52 m) wide, 35 ft (10.7 m) high (three-story) exterior panels carry the two upper floors and roof (see Fig. 6).

Wall panel size and shape can be af­fected by the details and locations of the vertical and horizontal panel-to­panel connections . Both gravity load transfer between panels and gravity and axial load combinations caused by lateral loadings or size of window openings can become the major fac­tors influencing panel structural di­mensions and connection design. Al­though, for most precast exterior bearing wall structures, it will be found that the gravity dead and live load condition will control structural dimensions.

When stemmed floor members, such as double tees , are used , the width of loadbearing walls or span­drels should module with the double­tee width. For example, for 12ft (3.66 m) double tees, walls should be 12, 24, or 36ft (3.66, 7.32, or 10.97 m) wide. Local precast concrete produc­ers should be contacted for their par­ticular module.

Inverted tee beams typically are used on interior spans. To minimize floor-to-floor dimensions , double tees are frequently dapped at interior beam lines and at exterior spandrels . Dap­ping is generally not necessary on ver­tical wall panels.

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Fig. 4. The brick-faced precast panels on the Barker Substation, Denver, Colorado are 36ft (11 m) tall.

Fig. 5. The Pol ice Administration Building at Philadelphia, Pennsylvania.

Fig. 6. Three-story panels supporting double tees.

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DESIGN CONSIDERATIONS In recent years, tremendous ad­

vances have been made in precast con­crete structural engineering technol­ogy . Greater knowledge regarding connections and wall panel design has made it possible to use architectural loadbearing precast wall panels more cost effectively. Solid panels, or pan­els with small openings, constitute "true" bearing walls as their major stress is in compression. Uniaxial bending from gravity loads is nor­mally only minor and incidental. With solid flat panels , load path locations can be determined easily.

As openings in the wall become larger, loadbearing concrete panels may approach frames in appearance and the concentration of load in the narrower vertical sections increases. In multistory structures this load accu­mulates, generally requiring reinforce­ment of the wall section as a column (at panel ends and at mullions between windows) designed for biaxial bend­ing due to load eccentricities.

Loadbearing panels and shear walls, generally, will be supported continu­ously along their lower surface. They may be supported by continuous foot­ings, isolated piers and grade beams or transfer girders. The bearing wall units can start at an upper floor level with the lower floors framed of beams and columns allowing a more open space on the lower levels.

When this is done, careful attention must be paid to the effective transfer of the lateral forces to the foundation. As with a vertical irregularity in any building in a seismic zone, the struc­tural engineer should make a careful assessment of the behavior and detail­ing. In multistory bearing walls, de­sign forces are transmitted through high quality grout in horizontal joints.

As in all precast construction , the transfer of vertical load from element to element is a major consideration . Differences in section, shape, archi­tectural features and unit stress result in a variety of solutions and types of connections.

Depending on wall section and foundation conditions, a loadbearing wall panel can be fixed at the base (shear walls for lateral forces) with the roof elements freely supported on the

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c a a y + u 0

Fig. 7. Building layouts in which loadbearin g panels can be used advantageously. (Note: Caution must be used with irregul ar plans in regions of moderate or high seismic risk.)

panel. Alternately, depending on the shape of the building, wall element flexural stresses can be reduced by pin connecting them at the foundation and providing shear wall bracings at the

ends or across the building to ensure lateral stability.

The design and structural behavior of exterior architectural precast con­crete bearing walls depends on the

Dill

Fig. 8. Plan view of possible locations of vertical cores with respect to loadbearing walls . (Note: Although the building core is an important element of the lateral force res isting system, it may be insufficient to handle the torsional effects of eccentrica lly app lied loads in some of these plans. A lso some plans have re-entrant corners that create plan irregularities.)

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panel shape and configuration. The de­signer should consider the following: • Gravity loads and the transfer of

these loads to the foundation -Vertical (gravity) loads are parallel to the plane of the wall, at an eccen­tricity influenced by the geometry of the wall, location of load, manufac­turing and erection tolerances.

• Magnitude and distribution of lat­eral loads and the means for resist­ing these loads using shear walls and floor diaphragms - Loads in the horizontal direction may be both parallel to and perpendicular to the plane of the wall.

• Location of joints to control volume change deformations due to con­crete creep, shrinkage and tempera­ture movements; influence upon de­sign for gravity and lateral loads, and effect upon non-structural com­ponents. Volume change effects can be evaluated using methods re­viewed in Chapter 3 of the PCI De­sign Handbook. 1 Particular caution must be exercised at load path tran­sitions, such as at the corners of a building where loadbearing and non-loadbearing panels meet or at re-entrant corners.

• Connection concepts and types re­quired to resist the various applied loads.

• Tolerances required for the structure being designed with regard to pro­duction and erection for both precast concrete units and connections, in­cluding tolerances for interfacing different materials.

• Specific requirements during the construction stage which may control designs, such as site accessibility.

Loadbearing or shear wall units should be the primary design consider­ation if one or more of the following three conditions exist:

1. There is inherent structural capa­bility of the units due to either their configuration or to sufficient panel thickness. The sculptural configura­tion of units often enables them to carry vertical loads with only a slight increase in reinforcement. For exam­ple, the precast concrete units may have ribs or projections that enable them to function as column elements for the structure. Ribs may be part of the architectural expression, or where

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flat exposed surfaces are required, ribs may be added to the back of panels for additional stiffness. Projections do not have to be continuous or straight, as long as no weak point is created within the units. Generally, there is lit­tle cost premium for sculptured panels when there is adequate repetition. Similarly, some flat panels (including sandwich wall panels) may be suffi­ciently thick to carry loads with only minor increases in reinforcement. Structural design of panels with insu­lation placed between layers of con­crete (sandwich panels) usually ig­nores the loadbearing capacity of the non-bearing wythe.4 If possible, the structural wythe of a sandwich wall panel should be kept on the tempera­ture stabilized side of the building to reduce thermal stresses due to temper­ature variation.

2. A uniform structural layout of the building facilitates distribution of lateral forces from wind or earth ­quake loads. Plus, this uniformity lends itself to repetitive, economic castings (see Fig. 7). This concept is difficult to employ if the load paths are continually changing. Cast-in­place topping on precast concrete floor units enable the floors to act as diaphragms, distributing lateral forces, reducing both individual wall unit loads and connections.

3. The building has a central core or bay designed to absorb lateral forces

e

ROLL -

CAST -IN-PLACE TOPPING

1-1----1-- PRECAST

and transfer them to the foundation (see Fig. 8). When the core creates a torsional irregularity, it should be sup­plemented by designing the perimeter panels as part of the lateral force re­sisting system. Plan irregularities cre­ated by the extended wings of the C and Z shapes in Fig. 7 are particularly problematic in moderate or high seis­mic risk areas. Because the core or bay provides the structural rigidity , panel-to-floor connections can remain relatively simple. A typical building core may contain an elevator lobby, elevators, stairways, mechanical and electrical equipment, and space for air ducts . While the core is being erected or cast, the precaster can proceed with the fabrication of the exterior wall units and install them as the shear wall or core is being constructed, often re­sulting in saving construction time.

Loadbearing wall panels used to construct the core are connected after erection to form composite T, L, U or box-shaped sections in plan. The main advantages of precast cores versus cast-in-place cores are surface finish quality, faster construction, and greater flexibility of the precast con­crete erection sequencing.

The three conditions do not preclude other situations where loadbearing panels or shear walls may be used.

Architectural precast panel design does not differ from two-dimensional frame design, once the panel is iso-

FLOOR TORSION RESISTED BY SPANDREL

TORSION RESISTED BY FORCES IN FLOOR CONSTRUCTION

MT = We

(a) (b)

Fig. 9. Loadbearing spandrel.

C = T= ~ 0

(c)

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LOAD I I I I I I I I I I

FACADE

Fig. 10. Principle of

diaphragm action in precast floors and

roofs.

lated and taken as a free body. Ac­cepted design procedures and code re­quirements apply. Perhaps the only design consideration difference is rec­ognizing the role precast concrete panel production and erection play in the overall design process. Similarly, usual accepted procedures and code requirements apply to the design of an individual precast concrete panel and its components.

When spanning horizontally, panels are designed as beams; or, if they have frequent , regularly spaced window openings as shown in Fig. I c, they are designed as Vierendeel trusses. A hor­izontal Vierendeel truss type panel lends itself to simple handling since it is shipped in its erected position, re­quires vertical load transfer connec­tions at each story level, and requires only minimum erection handling and erection bracing.

A two-story vertical panel requires additional erection handling because it needs to be rotated during erection, and because it demands more sophisti­cated erection bracing. When the pan­els are placed vertically, they usually are designed as columns, and slender­ness should be considered (see Sec­tions 3.5 and 4.9 of the PCI Design Handbook, Fifth Edition'). If a large portion of the panel is a window open­ing, as shown in Fig. 1 b, it may be necessary to analyze the member as a rigid frame.

Loadbearing spandrel panels are es­sentially perimeter beams, that may extend both above and below the floor surface, and transfer vertical loads from the floor or roof to the columns. Except for the magnitude and location of these additional vertical loads, the design is the same as for a non-load-

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PLAN

FLOOR UNITS

bearing spandrel. Loadbearing span­drels are either !edged, pocketed, or have individual or button haunches (also known as spot corbels) to sup­port floor/roof members. Steel shapes and plates may be cast in to reduce haunch height and, therefore, floor-to­floor height. Non-loadbearing (clo­sure) spandrel panels may have much the same cross section as loadbearing spandrels without ledges, pockets, or haunches.

Loadbearing members loaded non­symmetrically may be subject to both internal and external torsion. If the re­sulting applied load is not coincident with the member's shear center, tor­sion will exist along the span of the member. A typical arrangement of spandrel and supported floor is shown in Fig. 9.

Torsion due to eccentricity must be resisted by the spandrel. When torsion is resisted in this manner in the com­pleted structure, twisting of the span­drel during erection must be consid­ered. Spandrels that are pocketed to receive stems of the double-tee floor or roof slabs decrease torsion stresses greatly, as well as minimize twist and eccentricity during erection. If torsion cannot be removed by floor connec­tions, the spandrel panel should be de­signed for induced stresses.

Non-prestressed reinforced concrete members subject to torsion should be designed in accordance with the appli­cable provisions of the ACI Building Code, Chapter lP Prestressed mem­bers subject to torsion should be de­signed in accordance with the applica­ble provisions of the PCI Design Handbook, Chapter 4 , 1 Design of Spandrel Beams,6 and the ACI Build­ing Code.

Precast building elements are com­monly reinforced with welded wire fabric , mild reinforcing steel or pre­stressing steel. Unless analysis or ex­perience indicates otherwise, both loadbearing and non-loadbearing pan­els should be reinforced with an amount of mild reinforcing steel, as specified in the appropriate building code, and be at least equal to p = 0.001.

Lateral loads applied normal to the wall are the result of wind or seismic forces , and are usually transmitted to vertical stiffening cores, shear walls, structural frames , or other stabilizing components by roof and floor mem­bers acting as horizontal diaphragms. This reduces the load on individual wall units and their connections (see Fig. 10). The connections between fa<;:ade elements and floor members are normally designed as hinges in the direction perpendicular to their plane.

Vertical continuity is achieved by providing connections at horizontal joints of vertical members. Columns should be braced at each level through a continuous load path to the diaphragm.

SHEAR WALLS In many structures, it is economical

to take advantage of the inherent strength and in-plane rigidity of exte­rior wall panels by designing them to serve as the part of the lateral load re­sisting system. Wails taking horizon­tal loads from the effects of wind or earthquakes are referred to as shear walls . Shear walls are used as the most common and economical lateral force resisting system and have been utilized widely in buildings up to 30 stories.

A shear wall system's effectiveness is dependent largely upon panel-to­panel connection design. A significant advantage of jointed construction is in the inherent ease of defining load paths through connections. As such, it is relatively easy to separate a precast concrete lateral force resisting sys­tem' s performance from that of the verticalloadbearing frame.

Shear walls are vertical members, which transfer lateral forces, in or par­allel to the plane of the wall, from su­perstructure to foundation. Thus, shear

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walls act as vertical cantilever beams. Shear walls are placed at appropriate locations within and around the build­ing perimeter according to the archi­tectural and functional design require­ments .7 The 1 ft (0.305 m) thick panels on the Sarasota County Judicial Center located in Sarasota, Florida, measure 21 x 15 ft (6.40 x 4.57 m). They serve as shear walls at the cor­ners of the building (see Fig. 11 ). Con­tinuous steel plate connections were cast into the corner panels to permit a welded connection at the vertical cor­ner joint.

Typically, a structure incorporates numerous walls, which can be used to resist lateral forces in both principal axes of the building. The portion of the total lateral force which each wall resists depends upon the wall ' s bend­ing and shear resistance capacity, the participation of the floor, and the char­acteristics of the foundation.

It is common practice to assume that floors act as rigid elements for loads in the plane of the floor, and that the de­formations of the footings and soil can be neglected. Thus , for most struc­tures, lateral load distribution is only based on the properties of the walls. Shear wall building design is per­formed in accordance with Sections 3.7 and 3.10 of the PCI Design Hand­book, Fifth Edition. 1

The importance of earthquake loads varies according to a project' s geo­graphic location. Many areas of the United States require structural analy­sis for potential earthquake forces in varying degrees of intensity. Concrete panels have the inherent strength to perform as shear walls with little or no additional reinforcement. It is impor­tant, however, that the connections be designed to transfer lateral forces, and also accommodate thermal movements and differential deflections (or cam­ber) . The ability to transfer lateral forces may be a panel's only structural purpose. But, it is more often com­bined with loadbearing capabilities.

Shear walls are economical because walls already required by the building layout [such as exterior walls, interior walls (see Fig. 12)8•9 or walls of the el­evator, stairway, mechanical shafts or cores] can become structural shear walls. Load transfer from horizontal

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Fig. 11. Shear wall at the corners of the Sarasota County judicial Center, Sarasota, Florida.

17'-o" 35'-o" r 1, r I

1 I r BUILDING BOUNDARY

I @ I I 1--R26'-0" l -~:-~ .. -

1-----

1-----

=~-- , 1 L I. -

, ® 0 I © 1 ®~ ® @ 'f ~ ~

t; ~ ~~~~~ u WALL = @~ ~ 1 ~~-- J~

J _ CD 0 ----~ __ g e--~ ----~ --x

I @ I I I I FLOOR FORCE ll I

l 35'-0" l 35'-0" 35'-0" 35'-0" l NOTE: 1 ft. 0.3048 m; 1 in. = 25.4 mm

FLOOR PLAN

Fig. 12. Building layout showing shear walls for a symmetrical condominium building plan (Ref. 8) .

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diaphragm to shear walls, or to eleva­tor walls, stairway cores or mechani­cal shafts, can be accomplished either via connections or by direct bearing. Whenever possible, it is desi rable to design shear walls as loadbearing pan­els. The increased dead load acting on the panel is an inherent advantage be­cause it increases the panel's resis­tance to uplift and overturning forces created by lateral forces.

The effect of cumulative loads on connections between panels must be considered, since these loads become a s ignificant factor in determining minimum panel dimension s. Shear

walls in precast concrete buildings can be individual wall panels or wall pan­els which are connected together to function as a single unit. Connected panels greatly increase shear resis­tance capacity.

Co nnecti ng lo ng lengths of wall panels together, however, can result in an undesirable build-up of volu me change forces. Hence, it is preferable to connect only as many units as nec­essary to resist in-plane shear forces and the overturning moment. Connect­ing as few units as necessary near the mid-length of the wall will minimize the volume change restraint forces.

In some structures, it may be desir­able to provide shear connections be­tween non-loadbearing and loadbear­ing shear walls in order to increase the dead load resistance to moments caused by lateral loads. However, in most cases, an exterior shear wall (or per imeter frame) system provi des more efficient and flexible floor plans than does an interior shear wall system because it eliminates the need for a structural core (see Fig. 13).

Furthermore, exterior shear walls do not affect the interior traffic flow or sight lines. The exterior walls provide the vertical strength and horizonta l

Design • slabs and wall panels should (a)

be shaped to create self-forming elements and require a minimum of temporary support and bracing

• crack control is critical to avoid leakage and damage to outside surface of precast concrete panel

• if precast shell and cast-in­place concrete are to act compositely bond ties should be considered - but must be positioned so that reinforcement can be placed

• develops lateral load resisting wall and column sections

VERTICAL SECTION

HORIZONTAL SECTION

Fig. 13. Exterio r shear wa ll system (or perimeter frame).

• reinforcement that passes across the interface should be adequate to support required forces using shear friction

VARIATIONS

Fig. 14. Interio r shear wa l l system.

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• must be reinforced to transfer horizontal and vertical shear forces without undue deformations

Production • placement of panel

reinforcing steel must be held to close tolerances to minimize cracking

• good system but precast concrete difficult to cast

Erection • temporary shoring is often

required

Variations • provide inserts in panel to

facilitate forming for cast-in­place concrete

(b)

(C)

PRECAST PERMANENT .. , FORMWORK (ONE-PIECE :· SPANDRELS MAY REOUIRE :! SUPPORT AND RESTRICT : PLACEMENT OF CONCRETE)

Fig. 15. Precast concrete un its serving as forms for cast- in­place concrete to t ie wa ll s, beams, and f loor together.

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connections to allow the entire wall to function as a single unit to mobilize dead load overturning resistance. In addition, they eliminate the need for exterior columns and beams.

In an interior shear wall system, the lateral forces are not transferred di­rectly to the foundation. Instead, the wall panels distribute the lateral forces to floor diaphragms, which, in turn, transfer them to a structural core or to the interior shear walls (see Fig. 14). Frequently, the shear wall panels are connected vertically and at the corners to form a structural tube that can­tilevers from the foundation, creating a stronger element than its indiviudal parts.

PRECAST CONCRETE AS FORMS FOR CIP CONCRETE

Architectural precast concrete units also may serve as forms for cast-in-place concrete. This applica­tion is especially suitable for com­bining architectural (surface aesthet­ics) and structural functions in loadbearing fa~ades, or for improv­ing ductility in locations of high seismic risk by using wet cast con­nections with high levels of rein­forcement at the joints. Continuity and ductility are achieved by casting in place the beams and columns using precast concrete loadbearing panels as the exterior form work.

The ductility of walls depends par­tially upon reinforcement locations. Ductile behavior is improved signifi­cantly if the reinforcement is located at the ends of the walls. This way, structurally inactive cladding can be­come a major lateral load (seismic and wind) resisting element. Seismic loads are resisted primarily by the building's central core and partly by the ductile concrete exterior frame. Basically , floor slabs act as diaphragms. Fig. 15 illustrates the use of cast-in-place con­crete to tie the walls, beams, and floor together. This can be an efficient sys­tem for providing lateral resistance in precast concrete buildings.

The four-tiered colonnade wrapping Liberty Square in Vancouver, British Columbia, is constructed of precast panels that double as formwork for cast-in-place concrete (see Figs. 16 and 17). The loadbearing capacity of

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the integrated precast and cast-in­place frame resulted in a substantially stronger wall - one that is st ruc­turally independent of the central core.

The Marathon Plaza, San Francisco, California, consists of two terraced

towers, nine and ten stories high (see Fig. 18). Precast wall panels were de­signed with their edges serving as forms for columns and spandrel beams, thus integrating the panels into a tube to resist lateral forces (see Fig. 19).

Fig. 16. Liberty Square, Vancouver, British Columbia.

Fig. 17. Panels serve as forms for cast- in-place concrete.

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Fig. 18. Construction of Marathon Plaza, San Francisco, California.

Fig. 19. Close-up of edges of panel.

The cast-in-place concrete structure of the 13-story Simmons Biomedical Research Building, Dallas, Texas (see Fig. 20), was cast into forms consist­ing of precast concrete column and spandrel beam covers (see Fig. 21).

CONNECTIONS Connections for loadbearing wall

panels are an essential part of the struc­tural support system. The stability of the structure depends upon them. Loadbearing panel connections should be designed and detailed in the same

102

manner as connections for other pre­cast structural members . It is desirable to design loadbearing precast concrete structures with connections which allow lateral movement and rotation, and to design the structure to achieve lateral stability through the use of floor and roof diaphragms and shear walls.

Connection methods include bolt­ing, welding, post-tensioning, grout­ing, or a combination of these tech­niques. The floor system may or may not have a structural topping. Design­ers are referred to an extensive treat­ment of design methods in Refs. 1 and

Fig. 20. Simmons Biomedical Research Building, Dallas, Texas.

2. Often, loadbearing walls have hori­zontal and/or vertical joints across which forces must be transferred.

Connections must comply with local codes whose provisions generally vary across North America . Connections may be subject to functional require­ments such as recessing for flush floors and/or exposed ceilings. Indi­vidual manufacturers have developed specific connections over the years be­cause they suit their particular produc­tion and/or erection techniques. How­ever, some basic connection concepts governing design , performance and material requirements can be formu­lated. No attempt has been made to size or detail individual pieces, weld­ing or anchorages of the connections shown in this discussion. Instead, this

Fig. 21. Panels serve double duty as formwork for co lumns and perimeter beams.

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is an engineering task required for each individual project.

Horizontal joints in loadbearing wall construction usually occur at floor levels and at the transition to foundation or transfer beams. These joints may connect floors and walls or wall units only. The principal forces to be transferred are vertical and hori­zontal loads from panels above and from the diaphragm action of floor slabs.

Horizontal joint and connection de­tails of exterior bearing walls are espe­cially critical , because the floor ele­ments usually are connected at this elevation, and since a waterproofing detail must be incorporated. Vertical joints may be designed so that the ad­jacent wall panels form one structural unit (coupled), or act independently. In addition to the vertical shear force transfer due to lateral loads, vertical joints also may be subject to shear

Design • if connection is on exterior

face of panel, it is susceptible to corrosion unless protected with mastic or grout

• hardware layout drwg. required for G.C.

• can be designed for horizontal shear and uplift; flexure in angle limits uplift capacity

Production • simple • embedded plates in wall

may need to be jigged level if cast top-in-form to avoid tilting

Erection • quick and easy • few tolerance problems if

embedded plates are wider than angle

• welding may be difficult when connection is below grade

• space under wall usually filled with grout

Variations • connections may be placed

on both sides of wall to develop nominal moment resistance

• angles may be bolted to wall and/or foundation

• plates may be used in place of connection angles

Fig. 22 . Wa ll to Foundation (WFl ).

September-October 1999

forces induced by differential loads upon adjacent panels.

The stability of the structure during construction must be considered when planning erection procedures. There­fore, temporary guying and/or bracing must be provided until final structural stability is achieved in the completed structure. This bracing design is the responsibility of the precast concrete erector and should be shown on a bracing plan prepared by the erector. Sometimes, the bracing plan requires review by the engineer of record and building officials.

vide uniform load paths for ten si le forces . Lateral connections can allow rotation (pin connections) or be rigid (moment connections), depending upon the structural system selected.

Wall-to-Foundation Connections

Connections that transfer vertical or lateral loads from panel to panel may differ accordi ng to the particular building. Gravity load transfer often can be achieved with simple weld plate connections because of co n­crete's inherent strength in compres­sion. Mechanical reinforcement splice connections may be required to pro-

Wall-to-foundation connections are used to tie loadbearing walls to the foundation (see Figs . 22 to 26). Any connection joining a wall panel to a foundation wall or a continuous foot­ing should provide a means of leveling and aljgning the panel. The attachment method also should be capable of ac­cepting the base shear in any direction. In cases where an interior core carries lateral loads, thi s may be accomplished with a simple welded connection.

Slab-to-Wall Connections

Slab-to-wall connections are made to join precast or cast-in-place concrete

2SHIM STACKS/PANEL

VARIATIONS

Design • shear resistance is achieved • capacity can be increased

by use of confinement reinforcement around sleeve and bars

Production • projecting dowels from panel

can cause difficulties in storing and transporting panel

• location and alignment of dowels is critical

Erection • grouting coordination

required • location and alignment of

sleeve is critical • no connection for panel

during erection; necessary to brace

• use grout under panel • alignment of panel must be

made before initial set of grout

• must weather protect sleeve to prevent ice, water or debris from filling cavity

Variations • sleeve may be placed in

panel to receive dowels from foundation

• grout can be pumped into sleeve after alignment or before panel erection

• proprietary sleeve systems

• use insert or coupler and add threaded dowel in the field to reduce production and transportion problems

• single dowel is most commonly used

Fig. 23. W all to Foundation (WF2).

103

floor or roof members to precast con­crete walls (see Figs. 27 to 30). Con­nections joining the slabs and walls may require load transfer or bearing, diaphragm action, and moment resis­tance. Secondary forces caused by temperature fl uctuations, long- term shrinkage and creep, and bending mo­ments induced by end restrai nts, are usually of minor importance.

Gravity transfer may be through a continuous ledge (corbel) or individual (spot) corbels or connection hardware.

Blockouts in wall panels can also be used to support floor members. Such pockets in wall panels or spandrels greatly decrease torsion stresses, and

Design • hardware layout drwg.

required for G. C. • size joint for welding access

Production • simple • insert must be jigged so that

bolt is plumb

Erection • quick and easy • allows vertical adjustment

without crane

Variations • bolt head may be welded for

tensile and shear capacity • plate may be eliminated but

adjustment becomes more difficult

• use embedded bolt and projecting sleeve nut

• insert may be in foundation

Fig. 24 . Wa ll to Foundation (WF3).

Design • two directional stiffness • headware layout drwg.

required for G.C.

Production • care required in jigging

angles in form to ensure proper alignment

Erection • may require temporary

bracing

Variations • if wall needs to be tied to

floor slab, weld reinforcement to connection

Fig. 25. W all to Foundation (WF4).

104

also minimize twist and eccentricity during erection. These pockets require substantial draft on their sides [ l/2 in. ( 12 .7 mm) every 6 in . (152 mm) depth] and should have at least 21h in. (64 mm) cover to the exposed face. More cover [3 in. (76 mm) minimum] is required if the exterior surface has an architectural fin ish.

(203 to 229 mm) in the surrounding area will make the difference.

In the case of a fine textured finish, there can be a light area (the approxi­mate size of the blockout) shown on the face of the panel due to differential drying. This will usually be apparent, despite the uniformi ty of the texture. The initial cure of the 21h to 3 in. (64 to 76 mm) of concrete versus 8 to 9 in.

When the slab functions as a di­aphragm, the connections must trans­mit diaphragm shear and chord forces to a structural core, thus reducing the load on individua l exterior wall or spandrel units and their connections. In those instances, simple welded con­nections can be employed to join pan­els. When the wall participates more actively in lateral or shear resistance, larger and more numero us welded co nn ectio ns are req uired. Flat , stemmed or hollow-core slabs may be used. When the slab is used with a composite topping, some connections

Design • develops moment resistance

at base • can be used to resist uplift

forces • no positive connection until

bar is tensioned • hardware layout drwg.

required for G. C.

Production • duct placement tolerance in

wall panel is critical • grout vents may be required

Erection • may require temporary

bracing • bar, duct and hardware

placement tolerance in foundation and wall panel is critical

• requires drypack to reach design strength prior to tensioning

• post-tensioning equipment necessary

Variations • shim under panel • bars may be coupled at top

of foundation • post-tensioned bar may or

may not be grouted

Fig. 26. Wa ll to Fou ndation (W FS).

2 SHIM STACKS/PANEL

VA RIATIONS

I SLEEVE

COUPLER ORYPACK

- FLOOR SLAB

POLYSTYRENE AROUNO BAR FOR FIELO ALIGNMENT

1JE.=F- POST-TENSIONING ~ BAR

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may be necessary to achieve stability of the structure during erection with the final diaphragm connection achieved using dowels from the wall to the topping.

Most designs result in some degree of continuity for these connections.

Design • welding at bottom of slab is

not recommended as excess restraint results

• no moment capacity • must consider eccentricity of

loads

However, a fully fixed connection is generally not desirable. The degree of fixity can be controlled by a judicious use of bearing pads in combination with clamping forces, or by welding to anchor plates pl aced in the floor members. Reinforcing steel can also

Design

be doweled, threaded, or welded to the walls.

Connections to shear walls along the (non-bearing) s ides of floor or roof slabs should be able to transmit lateral loads and should either allow some vertical movement to accommodate

COIL INSERT AND FIELD PLACED ROO

• minimizes eccentricity of load on wall

• axial shortening of slab due to volume change should be considered when designing depth of recess

• top connection transfers horizontal shear forces or provides nominal torsion restraint for spandrel

VARIATIONS

• pocket dimensions and tee end must be planned so that slab can " swing" into place; pocketed connection should not be used at both ends of slab Production

• special forming required for corbel

• corbel may be precast and set in form

Erection • quick and easy • allows adequate tolerances • temporary bracing may be

necessary

Variations • steel corbel; may use inserts

in panel to position angle while welding

• flag shaped plate (g) welded to embedded plate in wall can be used in hollow-core joints

• variation of (d) and (g) , dowel may be in topping

Fig. 27 . Slab to Wa ll (SWl ).

September-October 1999

OR

(c)

SLOTTED INSERT

REINFORCEMENT OR ~THREADED INSERT ~---· J (DOWEL)

(e)

(f)

• top connection similar to connection SW1 (a) or (b) may be used

Production • minimum of embedded

hardware • special forming required to

allow stems to fit into pockets

• pockets in wall difficult to locate and form , usually do not follow tee taper

• pockets require adequate tolerance

• pocket may telegraph through gray conrete; exterior surface finish, e.g. retarded, sandblasted will help conceal

Erection • do not drypack pocket

around tee stem to allow stem freedom to rotate

• for ease of erection, pockets should not be used at both ends of slab

Variations • pocket may be at top of

panel

Fig. 28. Slab to Wal l (SW2 ).

VARIATION

......

. ·.· .•. I . , .. ~:

I •

: - - 2-112 1N. MIN.

.• ·

105

Design • develop a rigid, moment

connection • avoid use of this detail at

both ends of slab to prevent excessive restraint

• rotation of wall elements and effects on bracing wall connections and volume changes must be considered

• arrangement of weld plates must allow for welding access

• avoid overhead welding, if possible

Production • plate jigging is necessary

since embed is top-in-form as cast

• steel congestion must be well thought out

Erection • welding must be completed

before setting panel above

Variations • wall corbel in lieu of angle

seat

Fig. 29. Slab to Wa ll (SW3).

Design • connection allows

movements caused by temperature changes

• positive horizontal force transfer

• connection (c) allows vertical movement by flexing of plate and welds

• connection (d) allows vertical movement through flexibility of double tee flange

Production • insert must be plumb and

true • washer must be oversize so

it does not bind in the slot • simple

Erection • quick and easy • tolerance problems

minimized • do not overtighten bolt in (a)

Fig. 30 . Slab to W all (SW4).

106

PRE-WELDED

THREADED INSERT IN WALL PANEL

VARIATIONS

~(d)

~

Design • continuity through the

connections • connection is concealed and

protected • no connection between walls

until splice sleeves or ducts are grouted

• sleeve and sleeve grout are proprietary

Production • hardware placement is

critical • projecting dowels can cause

difficulties in storing and transporting panels if dowels project from bottom of panel

Erection • may be necessary to heat

grout in cold weather • temporary brace required • requires a grout crew in

addition to setting crew

Variations • sleeve connector can be

placed in either upper or lower panel - upper panel is preferred

Fig. 31. W all to W al l (WWl ).

Design • can be used to withstand

uplift forces • connection is hidden and

protected • connection is not developed

until tensioning is completed (bars are anchored)

Production • duct and hardware

placement in panels is critical

• tolerance on slab length critical

• thin panel outer lip projection subject to damage during handling

Erection • temporary bracing is

required • drypack, tensioning,

grouting sequence may limit erection to one story at a time

• grouting requires care to ensure complete filling

Variations • bars may or may not be post­

tensioned

Fig. 32. W all to W all (WW2).

• ..

I ~

: .il-v GROUT

)\-V SPLICE •• SLEEVE

;~- DRYPACK

•. • ~ POLYURETHANE FOAM AND SHIMS

FILL SLEEVE WITH GROUT PRIOR TO SETTING BAR IN PLACE

I VARIATIONS

SPIRAL DUCT SHEATHING

THREADBAR COUPLER

- SHIM & DRY PACK

GASKET AT SLEEVE GROUT NOT SHOWN FOR CLARITY

SPIRAL DUCT SHEATHING

THREADER COUPLER THREADER

GROUT TUBE WI NIPPLE GROUT TUBE PLUG

~ill.lcANCHOR NUT POCKET ANCHOR PLATE WI VERTICAL GROUT

~VENT HOLE SHIM & DRYPACK

GROUT NOT SHOWN FOR CLARITY

PCI JOURNAL

Fig. 33. Welded Alignment

Design • good shear transfer • rigid connection • possible volume change

restraint problems

Production • simple • face of panel to face of plate

dimension is critical

Erection •· • quick, easy . · ~ •. • ample adjustment allowance

Variations • various embedded plates or

shapes may be welded together

• one side could be bolted with slotted or oversized hole

(WA2). 1....----------------------l

camber and deflection of the floor units, or be designed to develop forces induced by restraining the units.

W all-to-W all Connections

Wall-to-wall connections are pri­marily intended to position and secure the walls, although with proper design and construction, they are capable of

carrying lateral loads from shear wall or frame action as well (see Figs. 31 to 33). The two locations of wall-to-wall connections are horizontal joints (usu­ally in combination with floor con­struction) and vertical joints.

The most practical connection is one that allows realistic tolerances and en­sures transfer of load between panels. It also is desirable that the connection

require no grouting or at the very least a minimum amount of field grouting.

APPLICATIONS

In the last 40 years, many tall struc­tures have been constructed with load­bearing architectural precast concrete window wall panels. Among them is the 20-story Mutual Benefit Building in Philadelphia, Pennsylvania, built in 1969 (see Fig. 34). The panels mea­sure 12ft high and 20ft wide (3.66 x 6.1 0 m) and each has four openings. The mullions are designed for column action. Spandrels are hidden behind dark glass panels permitting an accent of vertical lines.

Loadbearing sandwich window wall panels for the 20-story, 300ft (91 m) tall One Hundred Washington Square office building in Minneapolis, Minnesota, are 13ft high and 10ft wide (3.96 x 3.05 m) (see Fig. 35).11 •12 They have a 16 in. (406 mm) interior wythe, 21h in. (64 mm) of insulation and a 3 in. (76 mm) exterior skin. The corner columns have cladding at the base and then serve as insulated formwork for cast-in-place concrete for the rest of the height.

,,,,,, ,,:,,,::'''' ,, ,:,,,,,,.,:,,,,, ,,,,:,,,::'''::~~~::''''1 ,,,,,,,,,,,,,,,,,,,l:llll:lllr,,

:,,,:::::::::::::::::::::::::ii::::::m ,::::::::::::w::::::::::m::iiiiiiiil 1111111111 ""'w:::::::::::::::m:m

Fig. 34. Twenty-story Mutual Benefit Life, Philade lphia, Pennsylvania.

Fig. 35. Window wall panels serve as elements of Vierendee l truss on One Hundred Washington Square office bui ld ing, Minneapo lis, Minnesota.

September-October 1999 107

Fig. 36. The 32-story Tannen

Towers, Atlantic City, New jersey.

Fig. 37a. The Boston College parking structure in Chestnut Hill , M assachusetts.

Fig. 37b. Early construction shot of the Boston College parking structure.

108

The 32-story Tannen Towers condo­minium project in Atlantic City, New Jersey, completed in 1987 uses portal frames at the base, and bearing walls in the upper levels (see Fig. 36). The building is subdivided from top to bot­tom by a central corridor. A row of 37 ft (11.3 m) long bearing walls, which are typically 8 in. (203 mm) thick , runs along either side of the corridor. The walls cantilever 11 ft (3.35 m) be­yond the face of the base structure on both sides of the building. To stabilize the structure, the design links pairs of bearing walls across the corridor with steel ties - back-to-back angles rein­forced with a continuous plate.

The majority of loadbearing build­ings built in recent years, however, have been less than ten stories.

A typical application of loadbearing spandrels is in parking structures. The Boston College parking structure in Chestnut Hill , Massachusetts, is an en­tirely precast/prestressed concrete building comprised of precast columns and beams with precast spandrels sup­porting double tees (see Fig. 37a). 13

An early construction shot of the park­ing structure (see Fig. 37b) shows the erection of the precast frame.

The Barnett Bank parking facility in Jacksonville, Florida, is part of a cam­pus serving as a national headquarters (see Fig. 38). The loadbearing span­drels, which are 40 ft (12.2 m) long, eliminate the need for cladding and framing and make use of the section properties of the framing members.

The six-story precast concrete Com­munity Service Building Garage in Wilmington, Delaware, fits in with the neighboring architectural landscape in color, style, and ornamentation (see Fig. 39). In addition, the building is designed to serve as the architectural base for a future nine-story office building. The cornice spandrel at the roof is over 4 ft (1.22 m) thick and weighs over 80,000 lbs (36290 kg) . The elevation adjacent to an existing building and invisible from the street is constructed of precast concrete loadbearing shear walls .

The architect for the Interlocken Of­fice Campus in Broomfield, Colorado, innovatively used the same molds to create three unique all-precast office buildings (see Fig. 40a). Architectural

PCI JOURNAL

Fig. 38 . Barnett Bank parking fac ility, jacksonville, Florida.

September-October 1999

Fig. 39. Community Serv ice Building Garage, Wilm ington, Delaware.

Fig. 40a. lnterl ocken Office Campus, Broomfield, Colorado.

Fig. 40b. Construction showing use of columns and spandrels in lnterl ocken Office Campus.

109

Fig. 41. Internal Revenue

Service office building, Oklahoma

City, Oklahoma.

loadbearing spandrels and columns minimized the contractor's time and risk in completing the core and shell (see Fig. 40b). The architectural exte­rior used an acid-etch finish with two colors and an ashlar stone formliner.

The all-precast 10-story Internal Revenue Service office building in Oklahoma City, Oklahoma, has 8 in. (203 mm) thick prestressed rectangu-

110

Jar spandrels supporting 44 ft long x 10 ft wide x 28 in. deep (13.4 m x 3.05 m x 711 mm) double tees (see Fig. 41).14

The horizontal mass of the Crescent VIII building in Crescent Town Center in Denver, Colorado, is broken by ex­pressed vertical pilasters (see Fig. 42a). The architect wanted to maxi­mize the window space while also

maintaining a heavier, substantial wall form. This was achieved with the de­tailing and implication of the beam­and-column look to the precast panels (see Fig. 42b).

Shepard's/McGraw-Hill World Headquarters in Colorado Springs, Colorado, has 12 in. (305 mm) thick acid etched window box panels sup­porting 32 in. (813 mrn) deep double­tee floor and roof members spanning 60ft (18.3 m), creating large column­free areas (see Figs. 43a and b).

Perhaps the single most important factor to influence the design of the 11-story Orange County Regional Service Center in Orlando, Florida, was energy consumption (see Fig. 44). To simplify construction, the ar­chitectural precast elements became structural, loadbearing units with a built-in eyebrow for sun screening. The loadbearing 12 ft wide x 12.5 high ft X 3ft deep (3.66 X 3.81 X 0.91 m) window boxes, with fully opera­ble windows recessed 3 ft (0.91 m) from the building exterior surface provide total shade on the glass dur­ing most of a typical summer day, when heat gain to the building inte­rior would peak.

The loadbearing window wall units on the Aurora, Colorado Municipal Justice Center are two stories high and weigh 20,000 lbs (9080 kg) each (see Fig. 45a). They are boldly detailed

Fig. 42a. The Crescent Town Center Campus in Denver Technological Center, Denver, Colorado, features a series of buildings designed around an open, park-1 ike crescent.

Fig. 42b. The use of integrated loadbeari ng/architectural spandrel panels erected in 8 weeks facilitated a tight construction schedule (The Crescent Town Center Campus, Denver, Colorado).

PCI JOURNAL

Fig. 43a. Shepard 's/McGraw-Hill World Headquarters, Colorado Springs, Colorado.

Fig. 43b. Construction showing window box panels supporting double-tee floor and roof members (Shepard 's/McGraw-Hill World Headquarters, Colorado Springs, Colorado).

Fig. 44. Loadbearing window

boxes on Orange County Regional

Service Center, Orlando, Florida.

September-October 1999

with bullnoses, cornices, and friezes (see Fig. 45b).

The all-precast Jefferson Avenue Parking Structure in Detroit, Michi­gan, features modules with punched openings measuring four stories high and 10 ft (3.05 m) wide. Doubling them vertically provides the total height (see Fig. 46a). This small mod­ule allows the fac;:ade to curve in re­sponse to the shape of the nearby Re­naissance Center. The precast concrete bearing wall system supports 4 ft wide x 12 ft long (1.22 x 3.66 m) hollow­core slabs and 10 ft wide x 55 ft long (3.05 x 16.8 m) double tees in various areas (see Fig. 46b).

The Denver Wastewater Manage­ment Building in Denver, Colorado, is an all-precast structure including the core and shear walls with loadbearing window wall and solid panels (see Fig. 47a) featuring highly articulated Art Deco detailing (see Fig. 47b). The six­story office tower is topped by two mechanical floors with a curved pedi­ment (see Fig. 47c).

Hardened criteria for the walls and roof of the Conrail Computer Technol­ogy Center in Philadelphia, Pennsyl­vania, included the ability to withstand the impact of an irregular object at 200 miles per hour (320 km/hr) and an equivalent explosive force (see Fig. 48). The use of 12 in. (305 rnm) thick, 40 ton (36 t) precast wall panels not only met these criteria, but in their ca­pacity as bearing walls, lessened the cost of the building.

The 10 x 30ft (3 .05 x 9.14 m) bear­ing walls of the gymnasium at the United States Olympic Training Cen­ter in Colorado Springs, Colorado are 13 in. (330 mm) thick insulated wall panels (see Fig . 49a). Lateral forces are resisted by shear wall action of the exterior interconnected wall panels (see Fig. 49b).

CONCLUDING REMARKS

Architectural precast concrete' s full potential as loadbearing walls can be realized when the entire design or de­sign/build team - architect, engineer of record, mechanical engineer, con­tractor, and precaster - has the op­portunity to develop a project jointly starting at the project ' s preliminary

111

Fig. 45a. Two-story window wall units supporting double tees for Aurora, Colorado Municipal justice Center, Aurora, Colorado.

Fig. 45b. Details on

loadbearing panels for Aurora Municipal

justice Center, Aurora, Colorado.

design stage. Finish types , shapes , repetitive use of efficient and econom­ical precast concrete modules, joint lo­cations, access or site restriction, erec­tion procedures and sequencing, all become important considerations for a project's successful completion. Prop­erly implemented, an early and contin­uing dialogue between the designers and precaster will ensure maximum product quality and appearance at a minimum installed construction cost.

The following additional benefits can be derived by using architectural precast concrete units as loadbearing walls: 0 Prefabrication combined with speed

of erection saves valuable overall construction time. Production of precast concrete components and site preparation can proceed simul­taneously. On-site labor cost is min­imized, and erection is possible in all kinds of weather. Construction is much faster with a fully integrated structure and skin system where loadbearing wall panels provide both structural support and architec­tural finish. Rapid enclosure allows earlier access by finishing trades. Faster completion reduces interim financing costs and results in earlier cash flows.

DLoadbearing wall panels become part of the structural framing. They

Fig. 46a. Jefferson Avenue Parking Structure, Detroit, Michigan.

Fig. 46b. Hollow-core units supported by walls in walkway areas Uefferson Avenue Parking Structure, Detroit, Michigan).

112 PCI JOURNAL

Fig. 47a. Construction of total precast Denver Wastewater Management Building, Denver, Colorado.

Fig. 47b. Closeup of Art Deco details in Denver Wastewater Management Building, Denver, Colorado.

form the supporting structure for floors and roof at the building perimeter. This generates interior space free of perimeter columns and interior bearing walls, provid­ing maximum floor plan layout flexibility. When a loadbearing wall panel building is erected, the architect and owner receive single­source responsibility for the build­ing shell. This reduces the number of subcontractors and minimizes trade coordination.

DElimination of separate structural frames from exterior walls results in savings far exceeding the minimal additional costs of increased rein­forcement and connections required for loadbearing units. This savings is most apparent in buildings with a large ratio of wall-to-floor area.

OPrecast concrete, manufactured in factory-controlled conditions as­sures the highest quality possible, thus ensuring a uniformly high qual­ity fa<;:ade in the desired shapes, col­ors, and textures. Greatest economy is achieved by using an integral ar­chitectural finish for both exterior and interior faces. Integral finishes not only result in a savings of mate­rial and labor, but also reduces the overall thickness of the exterior wall. This permits maximum inte­rior space utilization . Precast con­crete panels resist weather and cor­rosion , requiring little or no maintenance. Their aesthetic versa­tility is virtually unmatched by any other material.

September-October 1999

0 Panels can be designed as recepta­cles and distributors for electrical, mechanical, plumbing and HV AC sub-systems, thereby decreasing trade overlap problems and elimi­nating the need for a separate wall cavity.

DLoadbearing window wall panels can inherently form deeply recessed

Fig. 47c. Final erection shot of Denver Wastewater Management Building, Denver, Colorado.

window frames to provide a high degree of s un shading. Thi s can minimize air-conditioning system costs by reducing thermal load. Also, the thermal mass of concrete and the possibility of incorporating insulation into a sandwich wall panel contribute to reducing heating and cooling costs.

113

nomical, attractive building. Such structures contribute significantly to the development of contemporary architectural philosophy - specifi­cally, a system in which the walls are actually doing the structural work they appear to be doing.

0 Architectural load bearing wall pan­els can be used effectively to reno­vate and rehabilitate old deteriorated structures.

0 Architectural load bearing wall pan­els can be used not only in all-pre­cast structures but also in structural steel framed structures and cast-in­place concrete structures.

Fig. 48. Conrail Computer Technology Center, Philadelphia, Pennsylvania.

0 Architectural precast concrete used innovatively for loadbearing walls makes possible a nearly unlimited range of aesthetic expression, new design concepts and more efficient and less costly construction.

0 Design flexibility for the precast ex­terior allows unique expressions while interior framing can be simple and standard. This provides an eco­nomical solution for structures with varying loading, fire and space plan­ning requirements . Precast con­crete ' s aesthetic flexibility simpli­fies changes in plane, relief, color, and texture. Wall panels can be cus­tom designed in desired shapes and sizes or may be selected from a vari­ety of standard sections depending

on the building ' s intended use and budget. For walls requiring repeti­tive fenestration, precast concrete can offer sculptured architectural ef­fects with maximum simplicity of structural design and minimum erection cost. Final results are lim­ited only by the designer's imagina­tion.

OPrecast concrete loadbearing wall units, comprising structural-aes­thetic functional features , provide the opportunity to construct an eco-

ACKNOWLEDGEMENT The author wishes to express his ap­

preciation to the reviewers of this arti­cle for their technical comments and helpful suggestions: Alex Aswad , Kenneth C. Baur, Paul Carr, Ned M. Cleland, John Garlich , Jim King , Charles LeMaster, George D. Nasser, Dennis L. Nemenz, H. W. Reinking, Sami H. Rizkalla , and Stanley J . Ruden.

Fig. 49a . U.S. Olympic Training Center, Colorado Springs, Colorado.

114

Fig. 49b. Erection of main roof system consisting of 401h in. (1 029 mm) deep lightweight double tees spanning 111 to 140 ft (34 to 43 m) from bearing wall to bearing wall (United States Olympic Training Center, Colorado Springs, Colorado) .

PCI JOURNAL

REFERENCES

1. PCI Industry Handbook Committee, 5. ACI Committee 318, "Building Code 10. Kulka, Felix, Lin, T. Y., and Yang, Y. PC! Design Handbook, Fifth Edition, Requirements for Structural Con- C., "Prestressed Concrete Building Precast/Prestressed Concrete Institute, crete (ACI 318-99)," American Con- Construction Using Precast Wall Pan-Chicago, IL, 1999. crete Institute, Farmington Hills, MI, els," PCI JOURNAL, V. 20, No. 1,

2. PCI Committee on Connections, Design 1999. January-February 1975, pp. 62-72. and Typical Details of Connections for 6. Klein, Gary J., "Design of Spandrel 11. Yee, Alfred A., "One Hundred Wash-Precast and Prestressed Concrete, Beams," Specially Funded R&D Pro- ington Square - Structural Design MNL 123-88, Precast/Prestressed Con- gram, Research Project No. 5, Pre- and Construction," PCI JOURNAL, V. crete Institute, Chicago, IL, 1988, 270 cast/Prestressed Concrete Institute, 29, No. 1, January-February 1984, pp. pp., and PCI Committee on Connection Chicago, IL, 1986. 24-48. Details, "Addendum to Design and 7. Cleland, N. M., "Design for Lateral 12. Pickersgill, David, "One Hundred Typical Details of Connections for Pre- Force Resistance with Precast Con- Washington Square - The Precaster' s cast and Prestressed Concrete," PCI crete Shear Walls," PCI JOURNAL, Story," PCI JOURNAL, V. 29, No. 1, JOURNAL, V. 40, No. 5, September- V. 42, No. 5, September-October January-February 1984, pp. 49-63. October 1995, pp. 46-57. 1997, pp. 44-63. 13. "Architectural Detailing and

3. Du Bois, Cornelius R. (Kin), Kipp, 8. Aswad, G. Gus, Djazmati, Basel and Precast/Prestressed Concrete Play Key Brian R., Muir, R. Wayne and Wolf, Aswad, Alex, "Comparison of Shear Role in Success of the Boston College Bruce R., "Design-Construction of Wall Deformations and Forces Using Parking Structure," PCI JOURNAL, United Bank Tower," PCI JOURNAL, Two Approaches," PCI JOURNAL, V. V. 41, No. 2, March-April 1996, pp. V. 35, No. 6, November-December 44, No. 1, January-February 1999, pp. 12-20. 1990, pp. 26-33. 34-46. 14. Matthews, Lisa, Himes, Herman C.,

4. PCI Committee on Precast Sandwich 9. Mackertich, Seroj, and Aswad, Alex, and Cronin, John L., "Design-Con-Wall Panels, "State-of-the-Art of Pre- "Lateral Deformations of Perforated struction of the Oklahoma City IRS cast/Prestressed Sandwich Wall Pan- Shear Walls for Low and Mid-Rise Building," PCI JOURNAL, V. 39, No. els," PCI JOURNAL, V. 42, No.2, Buildings," PCI JOURNAL, V. 42, 3, May-June 1994, pp. 12-25. and 3, March-April 1997 and May- No. 1, January-February 1997, pp. June 1997, pp. 92-134 and pp. 32-49. 30-41.

September-October 1999 115